Conventional (oxygenate-free) mogas (gasoline sold at the pump for road use) has been largely replaced by ethanol-containing gasoline in the United States; Canada, Europe and other countries are also mandating the use of oxygenates in gasoline. Currently, alcohols are favored to supply the mandated levels of oxygen in the blended fuels as environmental problems have arisen with respect to other oxygenates such as ethers. Ethanol is the alcohol most frequently used in view of its economics and availability from agricultural sources.
As explained in U.S. Pat. No. 6,258,987 (Schmidt), the ethanol is not usually blended into the finished gasoline within the refinery because the ethanol is water soluble. As a consequence of this solubility, an ethanol-containing gasoline can undergo undesirable change if it comes in contact with water during transport through a distribution system, which may include pipelines, stationary storage tanks, rail cars, tank trucks, barges, ships and the like: absorbed or dissolved water will then be present as an undesirable contaminant in the gasoline. Alternatively, water can extract ethanol from the gasoline, thereby changing the chemical composition of the gasoline and negatively affecting the specification of the gasoline, possibly leading to regulatory violations since the government may require a certain oxygenate content in the gasoline sold at the pump. Government regulation in the U.S., for example, has until recently limited the oxygen content of gasoline to 4.0 wt. % while also requiring that reformulated gasolines contain at least 1.5 wt. % of oxygen, resulting in the gasoline known as E10 when ethanol is used as the oxygenate at nominally 10 vol %. More recent regulations propose a grade known as E15 for newer vehicles and other grades are also on sale, for example, E85, for use in multi-fuel engines.
In order to avoid contact with water as much as possible, ethanol-containing gasoline is usually manufactured by a multi-step process in which the ethanol is incorporated into the product at a point which is near the end of the distribution system, e.g. at the product distribution terminal, “at the rack”. More specifically, gasoline which contains a water soluble alcohol such as ethanol, is generally manufactured by producing an unfinished and substantially hydrocarbon precursor subgrade or blendstock usually known as a Blendstock for Oxygenate Blending (BOB) at the refinery, transporting the BOB to a product terminal in the geographic area where the finished gasoline is to be distributed, and mixing the BOB with the desired amount of alcohol at the terminal.
As ethanol is typically blended at the distribution terminal and not at the refinery gasoline blend header, problems arise in the operation of the overall manufacturing and distribution process. Ethanol-free gasoline is typically produced within a refinery as a finished product which fully meets all necessary specifications for sale as an ethanol-free product. This finished gasoline can be manufactured to fit the required product specifications very precisely because analytical data for the product can be obtained during the manufacture (aka gasoline blending) process and used to control the blending process. As a consequence, manufacturing costs are kept to a minimum because expensive blendstocks are usually not wasted by exceeding specifications. Unfortunately, this type of precise manufacturing control is not possible for blending configurations where the final commercial grade ethanol-containing gasolines are prepared by mixing a non-ethanol containing subgrade blend manufactured at a refinery with ethanol at a location remote from the refinery.
Octane is a key gasoline specification which typically constrains production. The octane response (increase) when mixing ethanol and the BOB is not constant, but is dependent on the composition of the BOB. Limitations in the capability to predict the response of octane to ethanol addition increases production costs by reducing the capability to both optimize gasoline blend planning (including gasoline component purchases and sales) and to optimize gasoline production when using feedback from online octane engines to control the blending operation used for the BOB.
The general problem which therefore requires to be solved is the control of octane during the gasoline blending since the volume of ethanol in the finished product is governed by regulation. The process analyzers used to measure the properties of the gasoline produced during the blending process at the refinery report the octane of the BOB but not that of the final product blended with ethanol which is made at the remote distribution terminal. Hence the octane rating of the with-ethanol product must be inferred from the BOB octane and the blending operation at the refinery to make the BOB must target the octane sufficiently above specification in order to ensure that the final product as blended with ethanol at the terminal will conform to specification; this reflects imprecision in the capability to predict the octane “boost” due to the ethanol addition. In order to avoid “octane give-away” or the manufacture of a BOB which has an uneconomic and excessively high octane rating, it would obviously be desirable to develop an approach which improves the precision of the octane prediction so as to enable the BOB to be blended at an octane rating which enables the finished with-ethanol specification to be predictably and reliably achieved.
There are five general categories of existing approaches to estimate the effect of ethanol on octane: (1) assuming a constant (or proportional to BOB octane) octane boost due to the effect of the ethanol, (2) assuming a volumetric or molar blend value for ethanol octane, (3) measuring the ethanol effect during each blend (by measuring BOB and with-ethanol octane) and adjusting the BOB octane target accordingly, (4) spectroscopic methods to estimate the with-ethanol octane from the BOB spectrum (determined either online or offline), and (5) composition-based models for volumetric ethanol octane blend values. In the approach disclosed in U.S. patent application Ser. No. 13/101,580 (counterpart of PCT/US2012/036277, Kelly), the BOB is manufactured at the refinery site in accordance with an empirical relationship, valid for that refinery site under typical manufacturing conditions, between (i) a property value of the BOB stream, e.g. octane, as determined by an on-site online process analyzer, and (ii) the corresponding property value for the final gasoline stream when blended with the required proportion of oxygenate and measured by the specification mandated test method. U.S. Pat. No. 6,258,987, mentioned above discloses approach (3).
US Patent Application 2010/0131247 (Carpenter) proposes to model the BOB subgrade using spectroscopic measurements and associating the subgrade characteristics in the model to the properties of the finished oxygenate-containing gasoline, an example of approach (4) above. While the use of the chemometric models described in this application represents one way to assure compliance of the finished gasoline with specification, the development of the required, highly detailed models is itself time-consuming and possibly subject to error arising from misinterpretation and correlation between the properties of the finished gasoline and those of the BOB subgrade. Chemometric models such as this are typically sensitive to the hydrocarbon composition of the BOB, and therefore have a limited range of validity and need to be refitted for different compositional envelopes. Also, it is impractical to embed a chemometric model into the models normally used for refinery or gasoline blending optimization because of the enormous number of data points that have to be accommodated in the chemometric model if the optimization model is to extend over a reasonably broad scope of refinery operating conditions.
The use of composition based models for estimation of the ethanol effect as in approach (5) is found in JP 4624142 B2 (JP2006/249309 A, Tanaka/Cosmo Oil), JP 2010/0229336 A (Tanaka/Cosmo Oil) and JP 2005/029761 A, Watanabe/Nippon Oil).
A relationship between BOB composition and final octane is recognized by Anderson et al (Energy and Fuels 24, 6576-6585) and SAE Technical Paper 2012-01-1274) in demonstrating that ethanol octane blends by mole with BOB (hydrocarbon) octane and cites a potential dependence of the ethanol octane molar blend value on BOB isoparaffin content, consistent with approach 2.